Hi there

Personally I would wait when Octane 3 will be released,as per Otoy announcement,new Octane 3 will support in future(3.x) will support OpenCL and this means you can use AMD/Intel GPU or CPU which supports OpenCL for rendering and you don't need to have Nvidia and CUDA...

But if you want to use right now,I would go with GTX770 or GTX780 which have best results in Octane,"new" GTX970 or GTX980 are still slower than older 780 in Octane,you can search on that on Overclock.net or on Otoy forum 

New GTX are hit(980) and miss(970 and their 4GB gate),if you are gamer then you are in win,win situation,but this depends what you want from yours GPU,I've went with ATI R9 290 which I bought due the better OpenCL performance as I'm using lots of SW and renderers which supports OpenCL and there ATI/AMD are bit better than Nvidia and performance and price ratio is bit better too,although new R9 390X(if will be available in summer should be beast too)

I've been owner of the numerous Nvidia cards,but after few issues with their drivers(with ATI/AMD drivers I've got too few problems) and mainly Poser crashes on start up etc I've went with AMD/ATI for now,I'm not fanboy as I've been owner of numerous Nvidia and ATI cards 

New GTX Titan-X seems have great results,but until I will not see their performance in Octane I would be very careful with their purchase and price can be too bit issue,but really depends on few other bits and pieces and then I would decide,but personally I would wait for Octane 3 and OpenCL support 

Hope this helps

Thanks,Jura